Residual Check Valve
Some master cylinders designed for drum brakes incorporate a residual check valve to keep a slight amount of pressure on the system after the brake pedal has been released. These residual check valves are used on drum brake systems to keep around 5psi up to 12psi of pressure on the wheel cylinders cup seals. They also resist the momentum of the fluid returning to the master cylinder. 
Since the valve is keeping constant pressure on the system any small leaks will result in fluid leaking out instead of air leaking in. The residual check valve keeps a constant pressure on the wheel cylinder cup seals. This valve is not used on many late model vehicles because the wheel cylinders incorporate a cup seal expander that keeps the cup seals firm against the wheel cylinder walls.
These valves are located inside the master cylinder port leading to the rear drum brakes. They consist of the check valve and a spring that allows the brake fluid to enter the master cylinder only until the designated pressure is reached. Once this occurs spring pressure closes the valve maintaining low pressure on the brake system.
